It is apparent that in future both static applications, such as wind power installations, in vehicles, such as in hybrid and electric vehicles, and in the consumer sector, such as in the case of laptops and mobile telephones, will make increasing use of new battery systems, on which very high demands in terms of the reliability, safety, performance and life thereof are placed.
Batteries with lithium ion technology are particularly suited to such tasks. They are distinguished by high energy density and low self-discharge, inter alia. By definition, lithium ion batteries comprise two or more lithium ion cells that are interconnected. Lithium ion cells can be interconnected by virtue of parallel or serial interconnection to form modules, and then to form batteries. Typically, a module comprises six or more cells.
DE 102009046567 A1 discloses a battery that is constructed from a plurality of battery modules, wherein the battery modules are monitored by means of a central battery management system.
As shown in FIG. 1, a conventional battery system 10 may have a battery management system 11 with a central controller 15, which communicates with a plurality of cell monitoring units 16 (“Cell Supervision Circuit”; CSC) that are each associated with a plurality of battery cells 14 or battery modules. In the text below, depending on the context, all of the battery cells 14 arranged in battery modules, with or without the associated battery management system 11, can be referred to as a battery. The battery cells 14 are grouped into battery modules, the precise split of the battery cells into the battery modules not being shown in FIG. 1. The battery management system 11 may be accommodated with battery cells 14 or battery modules in a shared housing (not shown). The battery modules may each have a separate housing. Arrangement of the battery cells 14 in battery modules can be used to achieve better scalability. In order to monitor the correct operation of the battery cells 14, the battery cells are monitored by the plurality of CSCs 16. In this case, a CSC 16 typically has two respective battery modules associated with it. A CSC 16 contains measuring electronics that monitor the voltage and further parameters. The information obtained by means of the CSC 16 is sent via a communication bus 18, for example a CAN bus, to a central controller 15 that evaluates the data from all battery cells 14 and, in the event of deviations from defined parameters, takes corrective action or if necessary opens the contactors 17, 19 and disconnects the battery system 10.
The controller 15 has a low voltage side or a low-voltage-side portion 22 with a microcontroller 23 and a high voltage side or a high-voltage-side portion 24 with a microcontroller 25. The low voltage side 22 and the high voltage side 24 are connected to one another via DC isolation 29. The low voltage side 22 is connected to a Hall sensor 27 for measuring the battery current, with the high voltage side 24 being connected to a shunt 26 for measuring the battery current. The controller 15 communicates with the vehicle electronics by means of the bus 28. The electrical connections 12, 13 are used for supplying power for a motor vehicle, for example, and/or for recharging the battery.
A further topology for cell monitoring is shown in FIG. 2. In this case, the CSCs 16 transfer their information via a daisy chain 32. A CSC 16 does this by routing its data packet to the next, which adds its information and in turn forwards it to the next CSC 16. Each CSC 16 contains a cell voltage measuring unit 21 (CVM). The line comprising the CSCs 16 arranged in daisy chain topology is in turn connected to the controller 15.
The monitoring of the cell voltages and also of the currents and the temperature for transgression of particular limit values is an essential safety factor in a battery system. ISO standards, particularly ISO 26262: Functional safety for E/E systems in motor vehicles, require a certain safety level ASIL (automotive safety integrity level) to be achieved.
In order to ensure functional safety for the battery system 10, the data from the CSCs 16 are evaluated and compared with one another both on the high voltage side 24 and on the low voltage side 22 of the controller 15 in the two redundant microcontrollers 23, 25. In this case, the high-voltage-side microcontroller 25 uses the total voltage of the pack, that is to say of all battery modules, and the total current that is measured by means of the shunt 26, for example. The low-voltage-side microcontroller 23 measures the voltage of the individual battery cells 14 and also the current that is ascertained via the Hall sensor 27, for example.
Current and voltage need to be checked at the same instant in order to be able to calculate plausible values. In order to be able to compare the values from the high voltage side 24 and the low voltage side 22, these data also need to be ascertained in parallel. In order to obtain a synchronous data base, the controller therefore simultaneously sends requests via the communication bus 18 to the CSCs 16, Hall sensors 27 and the shunt 26, which are then reported back, ideally simultaneously.
In order to comply with the high ASIL level, the controller has numerous security and control functions that include self-monitoring of the controller 15, inter alia. In addition, the controller monitors the CSCs 16. In order to uphold the high ASIL level, the controller 15 needs to undergo elaborate programming in order to be able to calculate the plausibility of the CSC data and thus to ensure the necessary functional safety. The data are captured by the same CSCs 16 both for the high-voltage-side microcontroller 25 and for the low-voltage-side microcontroller 23 of the controller 15. The controller needs to synchronize both microcontrollers 23, 25 to one another.
However, the synchronization of the data presents a problem that can be solved only with a high level of involvement. For example, the measuring electronics never report back data in precise synchronism, which means that values from different instants, for example the voltage from a CSC 16 with the current from the shunt 26 or from the Hall sensors 27, are offset against one another. Furthermore, these values are also compared with asynchronous values when the values calculated in the high-voltage-side microcontroller 25 are compared with the values calculated in the low-voltage-side microcontroller 23. The resultant discrepancies need to be corrected with a high level of computation involvement in the controller 15, which entails a high level of software involvement. Very complex computation models need to be applied in order to plausibilize the safety of the system, and these have to undergo elaborate programming. Known topologies for monitoring battery cells require a high level of functional safety and hence a high ASIL level and therefore not a low level of software involvement.